1. Field of the Invention
The invention relates to a sprayer device, usually called a sprayer bowl, for spraying a liquid coating product in the form of fine droplets; it applies to spraying any type of liquid paint or varnish, for example. The invention also relates to any rotary head sprayer equipped with this kind of sprayer device. The invention is preferably applied to a sprayer with a very high rotation speed, typically of the order of 50,000 to 100,000 revolutions per minute.
2. Description of the Prior Art
Many types of coating product sprayer device in the form of bowls or bells rotating at high speed are known to the person skilled in the art. The product to be sprayed flows over the interior surface of the bowl and, due to the effect of centrifugal force, reaches the rim of the bowl from which it is sprayed in the form of fine droplets. The bowl is usually shaped so that the surface facing the object to be coated is globally concave, often frustoconical. For example, the document EP 0951942 describes a bowl which has a frustoconical distribution surface.
The document U.S. Pat. No. 6,926,31 describes an externally charged rotary electrostatic device for dispersing a liquid in the form of threads that form textile fibers as the liquid solidifies in the air through dispersion and evaporation of its more volatile constituents. The fibers are formed beyond the rotary device because the liquid is subjected to an electric field created between two electrodes situated downstream of the rotary device. The document U.S. Pat. No. 6,926,31 does not relate to electrostatic spraying of liquid coating products. Moreover, the rotation speed is insufficient to spray the liquid due to the effect of centrifugal force alone. The liquid is in the form of a thin layer when it enters the area in which the electric field is present.
The person skilled in the art knows that to obtain a fine and regular spray, the regularity with which the liquid spreads over the distribution surface up to the spraying edge is of particular importance. In particular, the volume of air entrained in rotation in the interior of the bowl can generate friction on the liquid film that spreads over the distribution surface, which has a favorable influence on spreading and spraying quality. However, nothing must interfere with imparting rotation to this volume of air, which is confined to the interior of the cavity of the bowl.
U.S. Pat. No. 2,658,472 describes a sprayer bell with liquid ejector nozzles inside it directed toward the distribution surface. The presence of these nozzles interferes with entraining in rotation the volume of air in the interior of the sprayer bell. Moreover, the nozzles deposit the liquid at a very short distance from the sprayer edge, which does not favor correct spreading of the liquid over said distribution surface before it is sprayed from the edge of the bell.
The invention proposes a rotary sprayer device having a combination of features that operate in conjunction to obtain a very fine spray, with an extremely close distribution of the diameter of the droplets, producing an improved appearance of the deposit, in particular in the case of a paint.
To be more precise, the invention provides a liquid coating product sprayer device including a tubular hub forming or accommodating a liquid feed pipe, a divergent centrifugal deflector adapted to be driven in rotation about a rotation axis and having a sprayer edge and a continuous distribution surface which has the overall shape of a trumpet bell and extends between the hub and the sprayer edge, and means for deflecting at least a portion of the liquid in a substantially radial direction toward an innermost portion of the trumpet bell shaped distribution surface.
It may be considered that this kind of distribution surface, combined with the fact that the cavity of the bowl is free of injectors because the liquid is deposited at the back of the bell, optimizes the action of the air rotating in the cavity of the bowl on the film of liquid that flows over the distribution surface. This achieves significant thinning of the layer of liquid (typically paint) on said distribution surface as it moves toward the sprayer edge, and a regular thickness of this layer. The thinning of the layer as it moves toward the sprayer edge can also be attributed to a considerable increase in the surface area of the basic distribution ring, in the direction perpendicular to the rotation axis, by reason of the xe2x80x9ctrumpetxe2x80x9d shape of the distribution surface, this increase in surface area being combined with the increase in the centrifugal speed of the liquid, related to the distance from the axis.
Note further that the trumpet shape of this surface favors good cleaning with the aid of a liquid cleaning product injected instead of the liquid to be sprayed. To be more precise, this surface has no discontinuity that can cause undesirable accumulation of certain constituents of the liquid to be sprayed, in particular certain pigments.
As the person skilled in the art knows, the sprayer edge can be a sharp edge or notched. Striations can be imprinted in this surface in the vicinity of the sprayer edge or disposed on the distribution surface.
In one embodiment, the profile of the distribution surface in half-section taken along the rotation axis of said rotary bowl is substantially exponential. This profile can also have the shape of a curve represented by a function with a fast rate of increase, such as y=x2 or y=xn, or any linear combination of functions of this type, or of the type y=ax, of which the exponential function is one particular instance. By xe2x80x9cfunction with a fast rate of increasexe2x80x9d is meant any function whose derivative increases with the variable. A hyperbolic section can also be adopted as the profile. Any linear combination of the above types of curves is equally suitable.
As a general rule, any rotary sprayer bowl combining a central feed (i.e. axial injection of liquid into the hub) with a distribution surface of the type described above falls within the scope of the invention.
According to another advantageous feature, the sprayer device further includes a distributor mounted in axial alignment with the hub, and extending as far as the back of the deflector, the distributor including a core forming an obstacle to the axial flow of the liquid and radial passages disposed to the rear of the core to direct at least the greater portion of the liquid flow toward the distribution surface.
The invention will be better understood and other advantages of the invention will become apparent in the light of the following description of one embodiment of a liquid coating product sprayer device, which description is given by way of example only and with reference to the accompanying drawing.